Conventionally, a radiation method of irradiating an object with radiation such as X-rays and detecting the intensity distribution of the radiation transmitted through the object to acquire the radiation image of the object is widely used in the field of industrial non-destructive inspection or medical diagnosis.
In the most popular radiation imaging method, a combination of a so-called “screen” which emits fluorescent light by radiation and a silver halide film is used.
In the above radiation imaging method, first, an object is irradiated with radiation. The radiation transmitted through the object is converted into visible light by the screen to form a latent image on the silver halide film. After that, the silver halide film is chemically processed to acquire a visible image.
A thus obtained film image (radiation image) is a so-called analog picture and is used for medical diagnosis or inspection.
A computed radiography apparatus (to be referred to as a “CR apparatus” hereinafter) which acquires a radiation image using an imaging plate (to be referred to as an “IP” hereinafter) coated with a stimulable phosphor as a phosphor is also being put into practice.
When an IP primarily excited by radiation irradiation is secondarily excited by visible light such as a red laser beam, light called stimulable fluorescent light is emitted. The CR apparatus detects this light emission using a photosensor such as a photomultiplier to acquire a radiation image and outputs a visible image to a photosensitive material or CRT on the basis of the radiation image data
Although the CR apparatus is a digital imaging apparatus, it is regarded as an indirect digital imaging apparatus because the image formation process, reading by secondary excitation, is necessary.
The reason for “indirect” is that the apparatus cannot instantaneously display the radiation image, like the above-described apparatus (to be referred to as an “analog imaging apparatus” hereinafter) which acquires an analog radiation image such as an analog picture.
In recent years, a technique has been developed, which acquires a digital radiation image using a photoelectric conversion device in which pixels formed from small photoelectric conversion elements or switching elements are arrayed in a matrix as an image detection means for acquiring a radiation image from radiation through an object.
Examples of a radiation imaging apparatus employing the above technique, i.e., having phosphors stacked on a sensor such as a CCD or amorphous silicon two-dimensional image sensing element are disclosed in U.S. Pat. Nos. 5,418,377, 5,396,072, 5,381,014, 5,132,539, and 4,810,881.
Such a radiation imaging apparatus can instantaneously display acquired radiation image data and is therefore regarded as a direct digital imaging apparatus.
As advantages of the indirect or direct digital imaging apparatus over the analog imaging apparatus, a filmless system, an increase in acquired information by image processing, and database construction become possible.
An advantage of the direct digital imaging apparatus over the indirect digital imaging apparatus is instantaneity. The direct digital imaging apparatus can be effectively used on, e.g., a medical scene with urgent need because a radiation image obtained by imaging can be immediately displayed at that place.
When the radiation imaging apparatus described above is used as a medical apparatus to detect the radiation transmission distribution of a patient as an object to be examined, a scattering ray removing member called a “grid” is normally inserted between the patient and a radiation transmission distribution detector (to be also simply referred to as a “detector” hereinafter) to reduce the influence of scattering rays generated when radiation is transmitted through the person to be examined.
A grid is formed by alternately arranging a thin foil of a material such as lead which hardly passes radiation and that of a material such as aluminum which readily passes radiation perpendicularly to the irradiation direction of radiation.
With this structure, radiation components such as scattering rays in the patient, which are generated when the patient is irradiated with radiation and have angles with respect to the axis of irradiation, are absorbed by the lead foil in the grid before they reach the detector. For this reason, a high-contrast image can be obtained.
If the grid stands still during imaging, the radiation reaching the lead in the grid is wholly absorbed including both the scattering rays and the primary rays of radiation. Since a distribution difference distribution corresponding to the array in the grid is formed at the detection section, a striped radiation image is detected, resulting in inconvenience in reading at the time of image diagnosis or the like.
A radiation imaging apparatus having a mechanism for moving the grid during imaging has already been placed on the market.
However, in the above-described conventional radiation imaging apparatus having a grid, a light receiving scheme using a sensor such as a CCD or amorphous silicon two-dimensional image sensing element is not used, and a signal read by a two-dimensional solid-state image sensing element is real-time electrical processing. For this reason, unlike an analog imaging apparatus or an indirect digital imaging apparatus such as a CR apparatus, the influence of vibration of the imaging section or the electromagnetic influence of the driving motor due to grid movement poses a problem.
More specifically, the vibration of the imaging section due to grid movement also vibrates the capacitor and signal lines. The weak electric capacitance varies, and noise is superposed on the radiation image.
Additionally, in the signal read by the sensor, when the motor is driven near the sensor to move the grid, the signal potential or control power supply potential varies due to the influence of electromagnetic noise, and noise is superposed on the radiation image.
The radiation image with noise superposed thereon may deteriorate, e.g., the medical diagnostic performance.
On the other hand, in the sensor such as a two-dimensional solid-state image sensing element, the amount of charges accumulated in the sensor increases in proportion to the signal accumulation time due to the influence of a dark current even in an unexposed state. The larger the amount of charges that do not contribute to an image signal becomes, the larger the noise added to the output image signal becomes.
Hence, imaging control is preferably optimized to make the accumulation time in the sensor as short as possible while eliminating the influence of grid vibration. Neither an apparatus nor system that implement such control are conventionally available.
In the conventional X-ray imaging apparatus, an X-ray beam is projected from an X-ray source through an object such as a medical patient to be analyzed. After the X-ray beam passes through the object to be examined, normally, an image intensifier converts the X-ray radiation into a visible light image, a video camera generates an analog video signal from the visible image, and the video signal is displayed on a monitor. Since an analog video signal is generated, image processing for automatic luminance adjustment and image enhancement is performed in an analog domain.
A solid-state X-ray detector having high resolving power has already been proposed, which is constructed by a two-dimensional array using 3,000 to 4,000 detection elements represented by photodiodes for each dimension. Each element generates an electrical signal corresponding to a pixel luminance of an X-ray image projected to the detector. The signals from the respective elements are individually read and digitized. Then, the signals are subjected to image processing, stored, and displayed.
A medical X-ray image need needs to have 4,096 or more grayscale levels. In addition, since the X-ray dose is preferably suppressed to reduce the exposure amount, the image signal amount is also limited. For this reason, an extremely noise-free system is required as compared to a general image sensing element.
In medical X-ray imaging, a grid is used to suppress the influence of X-ray scattering. A fixed grid is generally unsuitable to with a solid-state X-ray image sensing element and poses a problem of aliasing, a system may be built using a movable grid.
As described above, a medical X-ray image sensing apparatus is required to be noise-free. A vibration caused by the movable grid can be a new noise source. The noise is generated by, e.g., the piezoelectric effect of a high-permittivity capacitor used in a circuit for generating a reference potential or simply because the parasitic capacitance in the read circuit varies due to the vibration.
To obtain the highest image quality, grid drive control, X-ray detector movement control, and X-ray detector driving method must be appropriately executed.